1. Field of the Invention
The present invention relates to a passive-type distance measuring apparatus that is provided with a distance measuring sensor which converts the incident light into an analogue signal.
2. Description of the Related Art
A conventional passive-type distance measuring apparatus, which is widely used as a passive-type AF system for AF cameras, divides the image seen through a predetermined distance measuring zone (e.g., a focus detection zone in the case of an AF camera) into two images by a pair of separator optical systems to be respectively formed as two separate images (right and left images) on a corresponding pair of line sensors (right and left sensors). Each line sensor includes an array of photodiodes (an array of photoelectric conversion elements). Each photodiode converts the received light into an electric charge and accumulates (integrates) the electric charge. Then the accumulated electric charges are read out of the photodiodes in order at every photodiode as a picture signal (voltage). In accordance with the picture signals output from the pair of line sensors, a predetermined distance measuring operation is performed to obtain data such as the amount of defocus or the object distance that is necessary for bringing the object into focus. However, if this passive-type distance measuring apparatus uses a distance measuring sensor which converts the incident light into an analogue picture signal, the accuracy of measurement deteriorates and thus a proper calculated distance value or amount of defocus cannot be obtained by a distance measuring calculation by merely converting an analogue picture signal output from the distance measuring sensor into a digital signal, so as to use the digital signal in a predetermined distance measuring calculation, if the brightness and/or the contrast of an object is low because, e.g., the resolving power of a low brightness portion of the digital signal is low.
The primary object of the present invention is to provide a distance measuring apparatus which makes it possible to improve the accuracy of measurement.
To achieve the object mentioned above, according to an aspect of the present invention, a distance measuring apparatus is provided, including a light receiving device having at least one array of photoelectric conversion elements, wherein each of the photoelectric conversion elements converts light received thereon into an electric charge and accumulates the electric charge so that each accumulated electric charge is output in order from each photoelectric conversion element as an electrical picture signal of the light receiving device; an A/D converter which converts the picture signal into digital image data; a converting device which performs a logarithmic transformation on the digital image data to replace the digital image data with sensor data; and an operation device which performs a distance measuring calculation in accordance with the sensor data.
In an embodiment, the distance measuring apparatus further includes a determining device which determines whether a calculated distance value obtained by the distance measuring calculation is effective, whether a contrast in a portion of the sensor data is equal to or higher than a predetermined contrast value and whether an object brightness is equal to or greater than a predetermined object brightness, in accordance with the sensor data; and a prohibiting device which prohibits the converting device from performing the logarithmic transformation for the digital image data. If the determining device determines that the calculated distance value is not effective, that the contrast in the portion of the sensor data is lower than the predetermined contrast and that the object brightness is equal to or greater than the predetermined object brightness, the prohibiting device prohibits the converting device from performing the logarithmic transformation for the digital image data, so that a second picture signal that is output from the light receiving device is converted into second digital image data without performing the logarithmic transformation for the digital image data, wherein each of the photoelectric conversion elements accumulates the electric charge again so as to output the second picture signal, and the operation device performs the distance measuring calculation again in accordance with the second picture signal.
Preferably, the distance measuring apparatus further includes a memory in which each of the sensor data is stored.
The A/D converter inputs each picture signal in series, to subsequently convert a current picture signal of each picture signal into first digital image data, wherein the converting device subsequently inputs the first digital image data to perform a logarithmic transformation on the first digital data to convert the first image data into sensor data while the A/D converter converts a subsequent picture signal of the picture signals into second digital image data.
When the A/D converter converts the picture signals into second digital image data, the converting device can simultaneously input the first digital image data and perform a logarithmic transformation on the first digital data.
After the A/D converter has finished converting all of the picture signals, the converting device performs a logarithmic transformation on the second digital image data corresponding to the final picture signal.
Preferably, the light receiving device is a passive AF sensor.
Preferably, the distance measuring apparatus is incorporated in an AF camera.
According to another aspect of the present invention, a distance measuring apparatus is provided, including a light receiving device having at least one array of photoelectric conversion elements, wherein each of the photoelectric conversion elements converts light received thereon into an electric charge and accumulates the electric charge so that each accumulated electric charge is output in order from each photoelectric conversion element as an electrical picture signal of the light receiving device; an A/D converter which converts the picture signal into digital image data; a converting device which performs more than one logarithmic transformation for the digital image data in different conversion ranges to replace the digital image data with corresponding more than one sensor data of different conversion ranges; and an operation device which performs a distance measuring calculation in accordance with at least one of the sensor data of different conversion ranges. If an effective calculated distance value cannot be obtained by the distance measuring calculation using a first one of the more than one sensor data of different conversion ranges, the operation device performs the distance measuring calculation again using another one of the more than one sensor data of different conversion ranges. A conversion range in which one of the more than one logarithmic transformation is performed for the first one of the more than one sensor data is different from a conversion range in which another one of the more than one logarithmic transformation is performed for the another one of the more than one sensor data.
The distance measuring apparatus can further include a determining device which determines whether an object brightness is equal to or greater than a predetermined object brightness from an integral time of the light receiving device. If the determining device determines that a calculated distance value obtained by the distance measuring calculation using the first one of the more than one sensor data is not effective and that the object brightness is equal to or greater than the predetermined value, the operation device performs the distance measuring calculation again using a second one of the more than one sensor data. A conversion range in which a second one of the more than one logarithmic transformation is performed for the second one of the more than one sensor data is narrower than a conversion range in which a first one of the more than one logarithmic transformation is performed for the first one of the more than one sensor data.
In an embodiment, the converting device converts the picture signal into the digital image data and performs only the first one of the more than one logarithmic transformation for the digital image data if the object brightness is equal to or greater than the predetermined value at the time the light receiving element converts the picture signal into the digital image data.
In an embodiment, the converting device performs the more than one logarithmic transformation for the digital image data in different conversion ranges in a predetermined range in which the voltage of one of the digital image data which has the highest object brightness among the digital image data is determined as a reference value.
In an embodiment, the A/D converter inputs each picture signal in series, to subsequently convert a current picture signal of each picture signal into first digital image data, wherein the converting device subsequently inputs the first digital image data to perform a logarithmic transformation on the first digital data to convert the first image data into sensor data while the A/D converter converts a subsequent picture signal of each picture signal into second digital image data.
When the A/D converter converts the picture signals into second digital image data, the converting device can simultaneously input the first digital image data and perform a logarithmic transformation on the first digital data.
After the A/D converter has finished converting all of the picture signals, the converting device performs a logarithmic transformation on the second digital image data corresponding to the final picture signal
Preferably, the light receiving device is a passive AF sensor.

Preferably, the distance measuring apparatus is incorporated in an AF camera.
According to another aspect of the present invention, an AF camera is provided, including a passive AF sensor having at least one array of photoelectric conversion elements, wherein each of the photoelectric conversion elements converts light received thereon into an electric charge and accumulates the electric charge so that the accumulated electric charges are output from each photoelectric conversion element as an electrical picture signal of the passive AF sensor; a converting device which converts the picture signal into digital image data and performs a logarithmic transformation for the digital image data to replace the digital image data with sensor data; and an operation device which calculates an amount of defocus in accordance with the sensor data.
The present disclosure relates to subject matter contained in Japanese Patent Applications Nos.11-236996 and 11-236997 (both filed on Aug. 24, 1999) which are expressly incorporated herein by reference in their entireties.